In the gas phase process for production of polyolefins such as polyethylene, a gaseous alkene (e.g., ethylene), hydrogen, co-monomer and other raw materials are converted to solid polyolefin product. Generally, gas phase reactors include a fluidized bed reactor, a compressor, and a cooler (heat exchanger). The reaction is maintained in a two-phase fluidized bed of granular polyethylene and gaseous reactants by the fluidizing gas which is passed through a distributor plate near the bottom of the reactor vessel. Catalyst is added to the fluidized bed. Heat of reaction is transferred to the circulating gas stream. This gas stream is compressed and cooled in the external recycle line and then is reintroduced into the bottom of the reactor where it passes through a distributor plate. Make-up feedstreams are added to maintain the desired reactant concentrations.
The properties of the polymer formed by such a process can be controlled to some extent by varying the operating conditions, including the operating temperature, comonomer amount, and type and quantity of catalyst. Such properties include the molecular weight of the polymer product, the molecular weight distribution of the polymer product, polymer density, and the flow index of the polymer product.
The properties of the polymer product as extracted from the reactor system, as well as in processed form for sale to customers, is also important. Typically, polymer product is extracted from the reactor and extruded into a more manageable form, such as pellets or bars. The flow index of a polymer product produced by a gas phase process using Cr based catalysts, including those containing an aluminum alkyl such as diethyl aluminum ethoxide (DEALE), tend to show a decrease or otherwise downward shift in the flow index (a net increase in molecular weight) when passed through an extrusion line, as compared to the flow index of granular resin taken directly from the reactor. This difference in flow index between the extruded material and the raw product, or flow index “shift”, is typically small, amounting to only a few units, e.g., <2 dg/min, for Cr-based catalysts. However, for some processes, the flow index shift is high, such that control of the polymer properties may be lost, as well as the possibility that the properties of the polymer itself are changed or sacrificed due to chain scission and/or recombination and cross linking. Therefore, it would be desirable to control the flow index shift to some extent.
In some cases it is also found that the flow index of polymer particles of different size fractions vary substantially. When this variation is very large, it is difficult to obtain reliable flow index data for the bulk material.